Computer specialist Didier Stevens put up a simple text advertisement on the Internet offering downloads of a computer virus for people who did not have any.

Surprisingly, he found as many as 409 people clicking on the ad saying "Is your PC virus-free? Get it infected here!" during a 6-month advertising campaign on Google's Adword, said the IT security expert.

"Some of them must have clicked on it by mistake. Some must have been curious or stupid," said Mikko Hypponen, head of research at data security firm F-Secure.

There was no virus involved, it was an experiment aiming to show these kind of advertising systems can be used for malicious intent.